Fluid storage tanks which are provided with an elastic diaphragm therein for the isolated storage of two fluids are well known and have particular application in the aerospace industry where they have been used for the storage and expulsion of a fluid as necessary. Most typically, the tanks are of spherical shape or cylindrical with hemispheric ends and are provided with a single elastic diaphragm fixed within the tank at or near a diametric plane of the tank. The diaphragm may serve to separate a liquid stored in the tank on one side of the diaphragm from a pressurant gas contained on the other side of the tank, although it can be used to separate liquids or gases. When additional pressurant gas is supplied to the tank through an appropriate inlet port, the diaphragm is moved to force fluid on the other side of the diaphragm out of the tank through an appropriate port provided therefor. Other types of fluid storage and expulsion systems include a storage tank with a dual seal piston and cylinder arrangement with a seal-leakage drainage port located between the independent seals.
The prior art systems have many disadvantages. Among the disadvantages of the single-diaphragm tank is the diffusion of one fluid into the other through the diaphragm material. Where gases are used to pressurize liquids in the single-diaphragm tanks, gas diffuses through the diaphragm material and saturates the liquid. Also, the liquid can gradually diffuse through the diaphragm to displace the volume reserved for pressurant gas.
Another disadvantage of a single diaphragm tank is that one failure of the diaphragm allows the direct mixing of the fluids which were to be maintained separate. A further disadvantage associated with spacecraft tanks is the considerable time and effort which must be spent during certification of the tanks for reuse while verifying diaphragm integrity with pressure drop tests. With regard to the piston type accumulators, seal leakage frequently results from piston seal erosion. The gas seals ride on dry cylinder walls and fail relatively quickly.
U.S. Pat. No. 4,784,181 discloses an expansion tank with a two part bladder-type diaphragm wherein liquid is stored in the bladder and pressurized gas is contained in the space between the diaphragm and the housing.
U.S. Pat. No. 4,826,045 discloses a fluid storage and expulsion system designed for aerospace applications which utilizes a single diaphragm with reinforcement rings and guide members for controlled collapsing movement of the diaphragm.
U.S. Pat. No. 4,796,676 discloses a storage tank with a secondary containment bladder. When a vacuum pressure is applied to the tank to mate the tank and bladder, an embossed surface of the bladder provides communication channels for air evacuation during the vacuum phase to reduce buckling and improve the seal between tank and bladder.
It is to be noted that all of the above are basically single-diaphragm tanks and have their attendant disadvantages.
U.S. Pat. No. 4,777,982 discloses a containment vessel with a single diaphragm separating wall composed of two outer layers and a porous middle layer. A leak or diffusion through the outer layers to the inner porous layer is communicated through a porous ring to an outlet. The three layers of the diaphragm limits its flexibility and the use of a clamping ring for connecting the separating wall to the container restricts its usage to low pressure applications.